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generator question #209063 03/05/13 02:27 AM
Joined: Jul 2004
Posts: 9,608
gfretwell Online Content OP
This from Short Circuit Via HotLine1


Would you be able to post the attached 1-line diagram in the General Discussion Forum so others can see and discuss the installation please?

[Linked Image from]

Topic Subject: 3 Generators at Dwelling

This 1-line diagram shows the electrical layout to back-feed a dwelling with three 20KW single phase generators. Iím looking for comments on the layout.

thanks, shortcircuit

Greg Fretwell
Work Gear for Electricians and the Trades
Re: generator question [Re: gfretwell] #209064 03/05/13 02:42 AM
Joined: Jun 2004
Posts: 1,273
Tesla Offline
Looks expensive.

My bias would be for a single Diesel gen-set with dual fuel capacity -- that was always kept warm -- and which was automatically tested once a month long enough to warm it up.

I'd have only one transfer switch -- on automatic -- and omit all of the sub panels and pin & sleeve gear.

I'd have a sub-panel with a cut-out switch that would shed load for non-essential loads. The presumption being that the bulk of the total load is never to be shed. Otherwise, I'd flip the priorities -- putting the all of the priority loads on one sub-panel -- itself subject to the automatic transfer switch.

I'd have an unswitched daughter panel(s) so as to have enough breaker spaces.

Super critical loads would warrant a dedicated UPS.

The dual fuels would be Diesel #2 and piped in natural gas.

I'm presuming that everything would be rigged to survive a tsunami and hurricane. (None of that Fukushima, 36" above sea-level installation nonsense.)

Re: generator question [Re: Tesla] #209070 03/05/13 10:17 PM
Joined: Jun 2004
Posts: 613
shortcircuit Offline
Tesla...that is a great idea.

In the above simple layout...generator #1 is for the most important loads and #2 and #3 are for areas of the property that don't need power all the time. This approach allows for shut down of generator #2 and #3 when not needed, thereby saving alot of fuel.

The idea behind the pin & sleeve is to allow connection to an alternate generator if another were to fail in the group of 3.

As for cost...lets say all equipment cost about $18K-$20K...this isn't that much more than one large generator and would save alot of fuel.

Thanks for the input.

Re: generator question [Re: gfretwell] #209085 03/06/13 01:20 PM
Joined: Aug 2002
Posts: 402
jdevlin Offline
Some comments for both options.

Can't you save the fuel by shutting the loads down for 2&3 with the single unit?

With multiple units and pin sleeve option you have the option the rewire unit#2 into #1 panel in the event of #1 failure?

Re: generator question [Re: gfretwell] #209089 03/06/13 08:51 PM
Joined: Apr 2002
Posts: 7,254
HotLine1 Offline
Depending on a load calc for the items that are considered 'required' you may want to re-think this from square one. A 200 amp ATS with load shed technology and a 20 KW natural gas, or propane, genset should solve this.

Many McMansions here use the above 200/20 Load shed & I have not heard any complaints.

As to your layout, I see no issues of NEC compliance; the only possible issue may be Article 400.7/400.8 cord uses, depending on details of the pin/sleeve setup.

Re: generator question [Re: gfretwell] #209090 03/06/13 11:08 PM
Joined: Jul 2004
Posts: 9,608
gfretwell Online Content OP
This does bring up an interesting question. Does a 10kva generator running at close to capacity use less fuel than a 20kva generator running at half capacity?

I suppose that might be a question for my niece, she is a Generac rep.

Greg Fretwell
Re: generator question [Re: gfretwell] #209091 03/06/13 11:16 PM
Joined: Jun 2004
Posts: 613
shortcircuit Offline
jdevlin...yes the pin and sleeve is for flexablity to connect 1 panel to another generator feeder if a gen-set were to fail.

Hotline...I too see some conflict with article 400 and cord uses. Maybe move the pin and sleeve outside for connection at the generators. They would only be live when the generators are feeding the dwelling.

Would a generator be considered a piece of "utilization equipment"...a bit of a stretch of the definition?

Re: generator question [Re: gfretwell] #209092 03/06/13 11:48 PM
Joined: Jun 2004
Posts: 1,273
Tesla Offline

Diesel engines are famed for having a pretty linear performance curve from idle up to full power.

Otto engines ( automobile engines, Mr. Otto's design scheme )are much, much, worse than Diesels across all ranges -- and particularly near idle.

An idling Diesel can run very, very, lean -- without losing it's 'flame'... below a certain threshold, Otto cycle engines don't fire off at all -- and just eject the vapors into the exhaust.


But as true as that is -- it's irrelevant.

The o n l y time that the back-up system is in play is during emergencies. Fuel cost is not a serious factor.

Reliability is critical. Diesels are much, much, more reliable than Ottos. They have no spark circuit -- and are inherently more robust. A pre-warmed Diesel doesn't even need glo-plugs -- particularly in Florida's weather.

A Diesel that starts up on distillate #2 can also accept methane as an auxiliary fuel. ( 20% liquid + 80% gas ) You can go to Youtube to see videos of such rigs running on the freeways of America.

Because of their lower first cost, Otto cycle engines running on Poco natural gas are t h e single most common power back-up that I've commercially installed.

Lastly, one 60 kW set-up is sure to be cheaper than three 20 kW set-ups. That law of economics is why the Poco uses monster machines. There is no other reason.

Re: generator question [Re: gfretwell] #209093 03/07/13 12:17 AM
Joined: Apr 2002
Posts: 7,254
HotLine1 Offline
Yes, the diesel is more reliable then gas/propane/natural. But, for resi, the favorite is gas. Initial costs and the convenience of not having a fuel truck (except for propane)

Hence, our data center has 8 diesel, 2 MW, 13.2KV, with 3800 gal. belly tanks for each unit. Shortcircuit and I are both in climates ranging from 100+ summers, to 0 winters, so a warm diesel is a six month luxury here. Bless the block heaters.

Greg: BTW, the diesels mentioned gulp 180 gal/hr at 85% load, and the load bank testing was very, very close to that figure.

Re: generator question [Re: gfretwell] #209094 03/07/13 12:40 AM
Joined: Nov 2002
Posts: 244
wewire2 Offline
Since the cord is going to be fixed wiring I don't think the cord
would fall under 590 Temporary installations even though it is for
temporary power so
400.8 seems like it would apply.
(1)"flexible cords and cables shall not be used for the fixed
wiring of a structure."
(2)"where run through holes in walls"
(4)"where attached to building surfaces."

Also, as I understand it, the ampacities listed in table 400.4 and 400.5
are the cord ampacity ratings. You can use these figures when derating
but for the termination requirements of 110.14(C)(1)you will ultimately need to comply with
table 310.15(B)(16) Even with 90 degree terminations #4
is still not sufficient for a 100 amp load.
Feedback on this appreciated.

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